The TOPS-20 operating system by Digital Equipment Corporation (DEC) is a proprietary OS used on some of DEC's 36-bit mainframe computers. The Hardware Reference Manual was described as for "DECsystem-10/DECSYSTEM-20 Processor" (meaning the DEC PDP-10 and the DECSYSTEM-20).

TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman (BBN) and shipped as a product by DEC starting in 1976. TOPS-20 is almost entirely unrelated to the similarly named TOPS-10, but it was shipped with the PA1050 TOPS-10 Monitor Calls emulation facility which allowed most, but not all, TOPS-10 executables to run unchanged. As a matter of policy, DEC did not update PA1050 to support later TOPS-10 additions except where required by DEC software.

TOPS-20 competed with TOPS-10, ITS and WAITS—all of which were notable time-sharing systems for the PDP-10 during this timeframe. TOPS-20 is informally known as TWENEX.

TENEX

TOPS-20 was based upon the TENEX operating system, which had been created by Bolt Beranek and Newman for Digital's PDP-10 computer. After Digital started development of the KI-10 version of the PDP-10, an issue arose: by this point TENEX was the most popular customer-written PDP-10 operating systems, but it would not run on the new, faster KI-10s. To correct this problem, the DEC PDP-10 sales manager purchased the rights to TENEX from BBN and set up a project to port it to the new machine. In the end, very little of the original TENEX code remained, and Digital ultimately named the resulting operating system TOPS-20.

PA1050

Some of what came with TOPS-20 was merely an emulation of the TOPS-10 Operating System's calls. These were known as UUO's, standing for Unimplemented User Operation, and were needed both for compilers, which were not 20-specific, to run, as well as user-programs written in these languages. The package that was mapped into a user's address space was named PA1050: PA as in PAT as in compatibility; 10 as in DEC or PDP 10; 50 as in a PDP 10 Model 50, 10/50, 1050.

Sometimes PA1050 was referred to as PAT, a name that was a good fit to the fact that PA1050, "was simply unprivileged user-mode code" that "performed the requested action, using JSYS calls where necessary."

The "EXEC" accomplished its work primarily using

  • internal code, including calls via JSYS
  • requesting services from "GALAXY" components (e.g. spoolers)

Command processor

Rather advanced for its day were some TOPS-20-specific features:

  • Command completion BLISS, and Pascal.

Commands

The following list of commands are supported by the TOPS-20 Command Processor. Operands were at times memory addresses. "TOPS-20 allows you to use 18-bit or 30-bit addresses. Some monitor calls require one kind, some the other; some calls accept either kind. Some monitor calls use only 18 bits to hold an address. These calls interpret 18-bit addresses as locations in the current section."

PCL language features

PCL includes:

See also SDF Public Access Unix System.

See also

  • Time-sharing system evolution

References

Further reading

  • Storage Organization and Management in TENEX. Daniel L. Murphy. AFIPS Proceedings, 1972 FJCC.
  • Implementation of TENEX on the KI10. Daniel L. Murphy. TENEX Panel Session, NCC 1974.
  • "TOPS-20 User's Guide ." 1988.
  • "DECSYSTEM-20 Assembly Language Guide." Frank da Cruz and Chris Ryland, 1980.
  • "Running TOPS-20 V4.1 under the SIMH Emulator."
  • Origins and Development of TOPS-20 is an excellent longer history.
  • Panda TOPS-20 distribution.
  • SDF Public Access TWENEX.
  • SIMH Simulator capable of simulating the PDP-10 and running TOPS-20.
  • Manuals for DEC 36-bit computers .
  • PDP-10 Software Archive.
  • 36-bits Forever.